The present invention relates to the communication of cable television signals, and more particularly to the communication of Motion Picture Experts Group (MPEG) video signals and Internet protocol (IP) signals in a standard digital transmission frame format, such as DS-3, over a hybrid fiber coax (HFC) network.
Cable television operators have rapidly been installing digital technology for use with their existing hybrid fiber/coax network. Such technology utilizes a digital set-top box, which converts the signals received over the HFC network for reproduction on a subscriber""s television set or other video appliance, such as a video tape recorder. Current digital set-top boxes operate, for example, in either a 64-QAM (quadrature amplitude modulation) or 256-QAM mode for downstream transmission and have a built-in radio frequency (RF) return path modem. These boxes enable a host of emerging interactive services to be provided, including Internet access and Video-on-Demand services. Future digital set-top boxes with a built-in cable modem will enable even more advanced services such as video streaming, IP telephony, high definition (HD) television, digital games, and the like.
In order to transmit HD signals to a subscriber via the existing HFC network, cable operators currently have only one choice. This is to replace the subscriber digital set-top box with a new set-top box that can receive off-air HD signals using 8-VSB (vestigial side band) modulation. This option is very expensive and is not practical, since the set-top box is not capable of receiving 8-VSB signals via existing HFC networks. Moreover, cable operators have a need to distribute IP data as well as audio/video services between a master headend and remote headends which are distributed throughout the cable network.
There are several options for providing the transport of IP data via an existing HFC network. One option is to outfit each of the cable operators""remote headend systems with a dedicated digital interface card, such as a DS-3 card that can handle full duplex IP data transport. DS-3 is a framing specification for digital signals in the North American digital transmission hierarchy. A DS-3 signal has a transmission rate of 44.736 megabits per second. DS-3 is used, for example, on T3 synchronous integrated services digital network (ISDN) lines. The advantage of providing a dedicated DS-3 interface card at each remote site is that the entire DS-3 frame""s payload carrying capacity is available for downstream and upstream IP data. A possible disadvantage is that the number of STS-1 (synchronous transport signal) time slots available for distribution of video/audio services on the synchronous optical network (SONET) ring is reduced by one. This problem can be solved, for example, by replacing the DS-3 interface with a 155 Mb/s rate OC-3 interface. Due to its high data rate, the OC-3 interface is more costly to implement than the DS-3 interface.
It would be advantageous to provide a method for transporting IP data using a DS-3 interface. It would be further advantageous to provide such a method that is fully compatible with existing HFC networks used for cable television communication.
The present invention provides a method having the aforementioned and other advantages. In particular, the present invention transports IP data within a DS-3 frame that also carries MPEG-2 programs. In this manner, one STS-1 time slot can be shared for full duplex transport of both MPEG-2 programs and IP data. Moreover, the technique of the present invention enables the available DS-3 payload to be maximized.
In accordance with the present invention, a method is provided for transporting variable length packets and fixed length packets of length N in standard digital transmission frames. Each frame comprises a plurality P of sections having a fixed length M, which is greater than the length N. In order to transport the variable length packets, a fixed length trailer is added to each of the variable length packets. Each variable length packet with the added trailer is segmented into an integer number of segmented packets of lengths N. Padding bits are added, if necessary, to provide the integer number of segmented packets. At least one of a header and trailer is added to each of the segmented packets and fixed length packets. This step expands each segmented and fixed length packet to an expanded packet length M. P of the expanded packets are then combined into a frame for transmission as a standard digital transmission frame.
The standard digital transmission frames may include Q parity bits in addition to the P fixed length sections. In such an implementation, Q parity bits are added to the P expanded packets to form the standard digital transmission frame.
The fixed length trailer is added to each of the variable length packets at a first protocol layer (e.g., an adaptation layer of a communication protocol). The header and/or trailer is added to each of the segmented packets and fixed length packets at a second protocol layer (e.g., an IP/MPEG layer of the communication protocol). The first protocol layer communicates with the second protocol layer to identify the last segmented packet corresponding to a variable length packet. This enables segmentation control fields to be set for the segmented packets at the second adaptation layer.
In an illustrated embodiment, the standard digital transmission frames comprise DS-3 frames of a digital communication network. The variable length packets comprise Internet protocol (IP) datagrams. The fixed length packets comprise MPEG video packets.
In the illustrated embodiment, the fixed length trailer includes a packet length field. A header is added to each of the segmented packets and fixed length packets to form the expanded packets, and includes a packet byte field, a segmentation control field and a sequence number field. The fixed length trailer can further include a cyclic redundancy code (CRC) field. The header can further include a configuration mode field. The fixed length trailer and the header can also each include an additional field (e.g., a reserved field) for providing optional features. Moreover, a trailer including a plurality of parity bytes can be added to each of the segmented packets and fixed length packets.
In a specific embodiment, the fixed length MPEG video packets are each 188 bytes in length. The variable length packets are segmented into 188-byte segmented packets. A header and trailer, together comprising 8 bytes, are added to the MPEG video packets and segmented packets to provide 196-byte expanded packets. Three of the expanded packets are combined together with 7 bytes of parity data to create DS-3 frames having a length of 595 bytes each.